What happened
On 23 August 2013, an AS332 L2 Super Puma helicopter, registration G-WNSB, was performing an approach to Sumburgh Airport when it impacted the sea. During the descent, the aircraft's rate of descent increased significantly, reaching 2000 feet per minute, while the airspeed dropped to below 30 knots.
Approximately twelve seconds before the impact, an automated voice alert commanded the crew to "CHECK HEIGHT". Although the commander acknowledged the alert, the aircraft failed to level off or slow down at the 300ft minimum descent altitude. The co-pilot noted a pitch-up tendency and a loss of airspeed, prompting a warning to the commander. As the crew became visual with the sea, the commander attempted a recovery by pulling up on the collective and pushing the cyclic forward. However, due to the critical lack of airspeed and altitude, the aircraft was likely irrecoverable and struck the water at 17:17:23 UTC.
The investigation
The AAIB examined the flight data, cockpit communications, and human performance factors involved in the approach. The investigation focused on the crew's monitoring of the descent profile and their use of standard operating procedures (SOPs). Investigators reviewed the aircraft's integrated flight and display system (IFDS) and analyzed the crew's communication patterns during the final stages of the flight.
Findings
- The crew's verbal communications during the descent did not strictly adhere to SOP requirements, which may have prevented the detection of deviations from the intended flight profile.
- The 'Two Communication' rule, intended to identify potential pilot incapacitation, was not applied, meaning incorrect calls went unchallenged.
- Ambiguities in the operator's manual regarding approach profiles made it difficult to identify when the vertical descent was deviating from a suitable path.
- High workload for the pilot not flying (PNF) during descent may have impacted the effectiveness of the instrument scan.
- The aircraft failed to level off at or above the 300ft minimum descent altitude despite an automated altitude alert.
- There was limited consideration of alternative landing sites as weather conditions deteriorated.